Method of producing an abrasive product containing diamond

ABSTRACT

A method of producing an abrasive product comprises providing a mixture of a mass of discrete carbide particles and a mass of diamond particles, the diamond particles being present in the mixture in an amount such that the diamond content of the abrasive product is 25% or less by weight, and subjecting the mixture to elevated temperature and pressure conditions at which the diamond is crystallographically stable and at which substantially no graphite is formed, in the presence of a bonding metal or alloy capable of bonding the mixture into a coherent, sintered product, to produce the abrasive product. The bonding metal or alloy is preferably a combination of a transition metal or a transition metal alloy and up to 40% by volume of the bonding metal or alloy of a second metal which is a stronger carbide former than the transition metal or the transition metal alloy, or an alloy of the second metal.

BACKGROUND TO THE INVENTION

THIS invention relates to a method of producing an abrasive productcontaining diamond and cemented carbide.

Cemented carbide is a material which is used extensively in industry fora variety of applications, both as an abrading material and as a wearresistant material. Cemented carbides generally consist of suitablecarbide particles such as tungsten carbide, tantalum carbide or titaniumcarbide, bonded together by means of a bonding metal such as cobalt,iron or nickel, or an alloy thereof. Typically, the metal content ofcemented carbides is about 3 to 35% by weight. They are produced bysintering the carbide particles and the bonding metal at temperatures ofthe order of 1400° C.

At the other end of the spectrum, ultrahard abrasive and wear resistantproducts are found. Diamond and cubic boron nitride compacts arepolycrystalline masses of diamond or cubic boron nitride particles, thebonding being created under conditions of elevated temperature andpressure at which the ultrahard component, i.e the diamond or cubicboron nitride, is crystallographically stable. Polycrystalline diamond(PCD) and polycrystalline cubic boron nitride (PCBN) can be producedwith or without a second phase or bonding matrix. The second phase, whenprovided, may be, in the case of diamond, a catalyst/solvent such ascobalt, or may be a carbide forming element such as silicon. Similarsintering mechanisms are utilised in PCBN synthesis with variouscarbides, nitrides and borides being common second phases.

PCD and PCBN have a far higher wear resistance than cemented carbides,but tend to be somewhat brittle. This brittleness can lead to edgechipping of the working surface which can present a problem inapplications where fine finishes are required. Furthermore, ultrahardproducts such as PCD and PCBN can generally not be directly brazed ontoa metallic support. They are therefore often sintered in combinationwith a cemented carbide substrate. The bi-layered nature of suchultrahard products can be problematic in terms of thermo-mechanicalstresses between the two materials: differential expansion and shrinkageon heating and cooling due to different thermal expansion coefficientsand elastic moduli can lead to crack formation or unfavourable residualstresses if the substrate and the ultrahard products are too dissimilar.Another potential problem of such bi-layered materials is that ofundercutting, i.e preferential wear of the less abrasion resistantcarbide support. Further, machining of ultrahard products is difficultand costly, where carbide products can be relatively easily ground tothe final geometry.

Efforts have been made to solve some of these problems.

U.S. Pat. No. 4,525,178 describes a composite material which includes amixture of individual diamond crystals and pieces of pre-cementedcarbide. The mixture is subjected to elevated temperature and pressureconditions in the diamond stable region, to create a compositepolycrystalline diamond body. The mixture uses precemented carbide andnot discrete carbide particles.

U.S. Pat. No. 5,045,092 describes a method of forming a cementedtungsten carbide article with embedded diamond particles. In thismethod, the embedded diamond particles are produced in situ.

European Patent No 0,256,829 describes a cemented carbide modified tothe extent that it contains up to 20% by weight of cubic boron nitrideparticles. The cemented carbide is preferably produced under cubic boronnitride synthesis conditions so that damage to the cubic boron nitrideis minimised.

European Patent No 0,583,916 describes a method of producing an abrasiveproduct comprising providing a mixture of diamond and discrete carbideparticles, the diamond particles being smaller than the carbideparticles and present in the mixture in an amount of more than 50% byvolume, and subjecting the mixture to elevated temperature and pressureconditions at which the diamond is crystallographically stable, in thepresence of a binder metal capable of bonding the mixture into a hardconglomerate.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of producing anabrasive product comprising:

-   (1) providing a mixture of a mass of discrete carbide particles and    a mass of diamond particles, the diamond particles being present in    the mixture in an amount such that the diamond content of the    abrasive product is 25% or less by weight; and-   (2) subjecting the mixture to elevated temperature and pressure    conditions at which the diamond is crystallographically stable and    at which substantially no graphite is formed, in the presence of a    bonding metal or alloy capable of bonding the mixture into a    coherent, sintered product, to produce the abrasive product.

The bonding metal or alloy is preferably a transition metal or atransition metal alloy, more preferably cobalt, iron or nickel or alloysthereof.

The bonding metal or alloy more preferably comprises a combination of:

-   (a) a transition metal or a transition metal alloy, preferably    cobalt, iron, or nickel or alloys thereof; and-   (b) up to 40% by volume of the bonding metal or alloy (i.e.    metal (a) plus metal (b)) of a second metal which is a stronger    carbide former than the transition metal or the transition metal    alloy, or an alloy of the second metal.

The metal (b) is preferably selected from the group consisting ofsilicon, titanium, zirconium, molybdenum, niobium, tungsten, vanadium,hafnium, tantalum, chromium, manganese, boron, beryllium, cerium,thorium, and ruthenium.

Preferably, the bonding metal or alloy comprises from 60% to 99.5%inclusive by volume of the metal (a) and from 0.5% to 40% inclusive byvolume of the metal (b).

The metal (a) is preferably provided in powdered form, but may also beadded in the form of an organic precursor or salt precursor that issubsequently pyrolised to result in finely dispersed metal.

The metal (b) may be provided in powdered form but may also be added inthe form of an organic precursor or salt precursor. Additionally, themetal (b) may be provided in the form of a non-stoichiometric carbide,nitride or boride or in the form of a stoichiometric carbide, nitride orboride where this is sufficiently soluble in the metal (a) such thatmetal (b) can migrate thorugh metal (a).

The metals (a) and (b) may also be provided in the form of an alloy ofthe metals (a) and (b).

The bonding metal or alloy, e.g the metals (a) and (b) may be mixed withthe carbide particles and with the diamond particles and the mixture maythen be sintered as such, or the mixture may first be cold-pressed toproduce a weak but coherent body prior to sintering.

Alternatively, the bonding metal or alloy, e.g the metals (a) and (b)may be supplied in the form of a separate layer adjacent to thediamond-carbide mixture and infiltrated during the high temperature/highpressure treatment step.

The diamond particles are preferably present in the mixture in an amountsuch that the diamond content of the abrasive product is from 10% to 18%inclusive by weight.

The diamond particles may be fine or coarse. The diamond particlespreferably have a particle size in the range of from 0.2 μm to 70 μminclusive, preferably less than 20 μm, more preferably less than 10 μm.

The bonding metal or alloy is preferably used in an amount of from 2% to20% inclusive by weight of the abrasive product, more preferably from 5%to 20% by weight of the abrasive product, most preferably less than 15%by weight of the abrasive product.

The carbide particles may be any carbide particles used in themanufacture of conventional cemented carbides. Examples of suitablecarbides are tungsten carbide, tantalum carbide, titanium carbide andmixtures of two or more thereof.

The carbide particles preferably have a particle size in the range offrom 0.1 μm to 10 μm inclusive.

The sintering of the mixture of carbide and diamond particles and thebonding metal or alloy preferably takes place at a temperature in therange of from 1300° C. to 1600° C. inclusive, and at a pressure from 40to 70 kbar inclusive.

This step is preferably carried out under controlled non-oxidisingconditions.

The sintering of the mixture of carbide and diamond particles and thebonding metal or alloy may be carried out in a conventional hightemperature/high pressure apparatus. The mixture may be loaded directlyinto the reaction capsule of such an apparatus. Alternatively, themixture may be placed on a cemented carbide support or a recess formedin a carbide support, and loaded in this form into the capsule.

In a preferred method of the invention, the carbide particles, thediamond particles, and the bonding metal or alloy have volatiles removedfrom them prior to sintering, e.g by heating them in a vacuum. Thesecomponents are preferably then vacuum sealed by, for example, electronbeam welding prior to sintering. The vacuum may, for example, be avacuum of 1 mbar or less and the heating may be a temperature in therange of 500° C. to 1200° C. inclusive.

The abrasive product produced by the method of the invention may be usedas an abrasive product for abrading materials, or as a wear resistantmaterial, particularly in tool components or inserts which consist of anabrasive compact bonded to a cemented carbide support. Typicalapplications include the cutting of wood and construction materials aswell as the machining of various non-ferrous metallic work pieces.

DESCRIPTION OF EMBODIMENTS

The crux of the invention is a method of producing an abrasive productby providing a mixture of a mass of discrete carbide particles and amass of diamond particles, and subjecting the mixture to elevatedtemperature and pressure conditions at which the diamond iscrystallographically stable and at which substantially no graphite isformed, in the presence of a bonding metal or alloy capable of bondingthe mixture into a coherent, sintered product. The diamond particles arepresent in the mixture in an amount such that the diamond content of theabrasive product is 25% or less by weight, preferably in the range offrom 10% to 18% inclusive by weight.

The abrasive product produced is, in effect, a cemented carbide whichhas been modified by the addition of diamond particles. The addition ofthese particles provides the cemented carbide with greater abrasive andwear resistant properties.

The abrasive product produced must be substantially free of graphite.The presence of any significant quantity of graphite reduces theabrasive wear resistant properties of the product. In producing theproduct, it is important that conditions are chosen which achieve this.

The sintering step is carried out in the presence of a bonding metal oralloy which preferably comprises a combination of (a) a transition metalor transition metal alloy and (b) up to 40% by volume of the bondingmetal or alloy of a second metal which is a stronger carbide former thanthe transition metal or transition metal alloy, or an alloy of thissecond metal.

As the carbide forming metals tend to react with the diamond particles,high amounts of such metals can result in excessive loss of the diamondphase and the formation of a high proportion of undesirable brittlephases. Thus, metal (b) is used in an amount up to 40% by volume of thebonding metal or alloy, i.e the total metal content, and this has beenfound sufficient to achieve a highly wear resistant product.

The presence of the metal (b) leads to improved bonding of the diamondgrains to the carbide matrix and thus to an improvement in theproperties of the abrasive product produced.

The invention will now be described in more detail with reference to thefollowing examples.

EXAMPLE 1

A powder mixture of 14.9 wt % diamond, 75.7 wt % tungsten carbide and9.4 wt % cobalt, all in the size range 1 to 2 micron, was thoroughlymixed in a planetary ball mill to achieve a homogeneous blend of thematerials. The blend was uniaxially compacted to form a coherent pellet.The pellet was loaded into a metal canister and subsequently outgassedunder vacuum at 1100° C. and sealed by electron beam welding. The sealedcontainers were loaded into the reaction capsule of a standard highpressure/high temperature apparatus and the loaded capsules placed intothe reaction centre of this apparatus. The contents of the capsule wereexposed to a temperature of approximately 1450° C. and a pressure of 50kbar. These conditions were maintained for 10 minutes. After completionof the treatment a well-sintered, hard and wear resistant material wasrecovered from the canister.

The abrasion resistance of the material was tested using a turning testwhere silica flour filled epoxy resin was machined using the followingconditions:

-   Sample format: 90° quadrant 3.2 mm thick-   Tool holder: neutral-   Rate angle: 0°-   Clearance angle: 6°-   Cutting speed: 10 m/min-   Depth of cut: 1.0 mm-   Feed rate: 0.3 mm/rev-   Test duration: 60 s

Under the given conditions the material exhibited a maximum flank wearwidth of 0.21 mm.

EXAMPLE 2

In order to assess the benefit of a more ‘active’ carbide-forming metal,in this case Cr₃C₂, the following mix was prepared using the method ofExample 1:

-   14.9 wt % diamond-   75.7 wt % tungsten carbide-   8.5 wt % cobalt-   0.9 wt % chromium carbide (Cr₃C₂,)

Using the same turning test as in Example 1 the material showed amaximum flank wear width of 0.11 mm.

EXAMPLE 3

A further sample was prepared to assess the benefit of chromium ascarbide-forming metal, this time introduced not as a carbide but asmetal:

-   14.9 wt % diamond-   76.0 wt % tungsten carbide-   5.7 wt % cobalt-   2.3 wt % nickel-   1.1 wt % chromium

Using the same turning test as in Example 1 the material showed amaximum flank wear width of 0.09 mm.

1. A method of producing an abrasive product comprising the steps of:(1) providing a mixture including a mass of discrete carbide particlesand a mass of diamond particles, the diamond particles being present inthe mixture in an amount such that the diamond content of the abrasiveproduct is 10 to 18% inclusive by weight; and (2) subjecting the mixtureto elevated temperature and pressure conditions at which the diamond iscrystallographically stable and at which substantially no graphite isformed, in the presence of a bonding metal or alloy capable of bondingthe mixture into a coherent, sintered product, to produce the abrasiveproduct; wherein the bonding metal or alloy comprises a combination of60–99.5% inclusive by volume of a first metal a) selected from the groupconsisting of transition metals and transition metal alloys, and 0.5–40%inclusive by volume of a second metal b) which is a stronger carbideformer than the metal a); and the metal b) is selected from the groupconsisting of silicon, titanium, zirconium, molybdenum, niobium,tungsten, vanadium, hafnium, tantalum, chromium, manganese, boron,beryllium, cerium, thorium, ruthenium, and alloys thereof.
 2. A methodaccording to claim 1 wherein the metal b) comprises chromium.
 3. Amethod according to claim 1 wherein the transition metal is selectedfrom the group consisting of cobalt, iron and nickel.
 4. A methodaccording to claim 1 wherein the metal (a) is provided in a formselected from powdered form and the form of an organic precursor or saltprecursor that is subsequently pyrolised to result in finely dispersedmetal.
 5. A method according to claim 1 wherein the metal (b) isprovided in a form selected from powdered form; the form of an organicprecursor or salt precursor; the form of a non-stoichiometric carbide,nitride or boride; and the form of a stoichiometric carbide, nitride orboride soluble in the metal (a).
 6. A method according to claim 1wherein the metal (a) and the metal (b) are provided in the form of analloy of the metal (a) with the metal (b).
 7. A method according toclaim 1 wherein in step (1) the bonding metal or alloy is mixed with thecarbide particles and with the diamond particles, and in step (2) themixture is subjected to the elevated temperature and pressureconditions.
 8. A method according to claim 1 wherein in step (1) thebonding metal or alloy is mixed with the carbide particles and with thediamond particles, and the mixture is cold-pressed to produce a weakcoherent body, and in step (2) the weak coherent body is subjected tothe elevated temperature and pressure conditions.
 9. A method accordingto claim 1 wherein in step (1) the bonding metal or alloy is supplied inthe form of a separate layer adjacent to the mixture of the mass ofcarbide particles and the mass of diamond particles, and in step (2) thebonding metal or alloy is infiltrated when the mixture is subjected tothe elevated temperature and pressure conditions.
 10. A method accordingto claim 1 wherein the diamond particles have a particle size in therange of from 0.2 μm to 70 μm inclusive.
 11. A method according to claim1 wherein the bonding metal or alloy is used in an amount of from 2% to20% inclusive by weight of the abrasive product.
 12. A method accordingto claim 1 wherein the carbide particles are selected from the groupconsisting of tungsten carbide particles, tantalum carbide particles,titanium carbide particles, and mixtures of two or more thereof.
 13. Amethod according to claim 1 wherein the carbide particles have aparticle size in the range of from 0.1 μm to 10 μm inclusive.
 14. Amethod according to claim 1 wherein in step (2) the elevated temperatureand pressure conditions are a temperature in the range of from 1300° C.to 1600° C. inclusive and a pressure of from 40 kbar to 70 kbarinclusive.
 15. A method according to claim 1 wherein step (2) is carriedout under controlled non-oxidising conditions.
 16. A method according toclaim 10 wherein the bonding metal or alloy is used in an amount of from2% to 20% inclusive by weight of the abrasive product.
 17. A methodaccording to claim 16 wherein the carbide particles are selected fromthe group consisting of tungsten carbide particles, tantalum carbideparticles, titanium carbide particles, and mixtures of two or morethereof.
 18. A method according to claim 17 wherein the carbideparticles have a particle size in the range of from 0.1 μm to 10 μminclusive.
 19. A method of producing an abrasive product, comprising thesteps of: providing a mixture including discrete carbide particles anddiamond particles, the diamond particles being present in an amount suchthat the abrasive product has a diamond content of 10–18% by weight; andsubjecting the mixture to elevated temperature and pressure conditionsat which the diamond is crystallographically stable and substantially nographite is formed, in the presence of a bonding metal or alloy toproduce the abrasive product; wherein the bonding metal or alloycomprises 60–99.5% by volume of a first metal a) selected from the groupconsisting of transition metals and transition metal alloys, and 0.5–40%by volume of a second metal b) selected from the group consisting ofsilicon, titanium, zirconium, molybdenum, niobium, tungsten, vanadium,hafnium, tantalum, chromium, manganese, boron, beryllium, cerium,thorium, ruthenium, and alloys thereof.
 20. The method of claim 19,wherein the first metal is selected from the group consisting of cobalt,iron, nickel, and alloys thereof.
 21. The method of claim 19, furthercomprising the step of providing the first metal in powdered form. 22.The method of claim 19, further comprising the step of providing thefirst metal in the form of a pyrolised organic precursor or saltprecursor.
 23. The method of claim 19, further comprising the step ofproviding the second metal in powdered form.
 24. The method of claim 19,further comprising the step of providing the second metal in the form ofan organic precursor or salt precursor.
 25. The method of claim 19,further comprising the step of providing the second metal in the form ofa carbide, nitride or boride that is soluble in the first metal.
 26. Themethod of claim 19, wherein the elevated temperature is 1300–1600° C.27. The method of claim 19, wherein the elevated pressure is about 40–70kbar.
 28. The method of claim 19, wherein the carbide particles areselected from the group consisting of tungsten carbide particles,tantalum carbide particles, titanium carbide particles, and combinationsthereof.
 29. The method of claim 19, wherein the bonding metal or alloyconstitutes 2–20% by weight of the abrasive product.
 30. A method ofproducing an abrasive product, comprising the steps of providing amixture including carbide particles having a particle size of 0.1–10microns and diamond particles having a particle size of 0.2–70 microns,the diamond particles being present in an amount such that the abrasiveproduct has a diamond content of 10–18% by weight; and sintering themixture at a temperature of 1300–1600° C. and a pressure of 40–70 kbarin the presence of a bonding metal or alloy to produce the abrasiveproduct; wherein the bonding metal or ahoy comprises 60–99.5% by volumeof a first metal a) selected from the group consisting of transitionmetals and transition metal alloys, and 0.5–40% by volume of a secondmetal b) selected from the group consisting of silicon, titanium,zirconium, molybdenum, niobium, tungsten, vanadium, hafnium, tantalum,chromium, manganese, boron, beryllium, cerium, thorium, ruthenium, andalloys thereof.
 31. The method of claim 30, wherein the diamondparticles have a particle size of less than 20 microns.
 32. The methodof claim 30, wherein the diamond particles have a particle size of lessthan 10 microns.
 33. The method of claim 30, wherein the sintering isperformed under controlled non-oxidising conditions.
 34. The method ofclaim 30, further comprising the step of removing volatiles from thecarbide particles, diamond particles, and bonding metal or alloy priorto sintering.
 35. The method of claim 34, wherein the volatiles areremoved by applying a vacuum pressure of 1 mbar or less at a temperatureof 500–1200° C.
 36. The method of claim 30, wherein the bonding metal oralloy constitutes 2–20% by weight of the abrasive product.
 37. Themethod of claim 30, wherein the bonding metal or alloy constitutes 5–20%by weight of the abrasive product.
 38. The method of claim 36, whereinthe bonding metal or alloy constitutes less than 15% by weight of theabrasive product.